This invention relates generally to a process for curing a polyimide layer, and more specifically to a process for the rapid, low temperature curing of polyimide layers and to a process for monitoring that curing.
Polyimides have become very widely used materials in applications requiring a protective coating, insulating layer, or the like. For example, in the semiconductor industry, polyimide coatings are used for arc suppression, dielectric passivation, interlayer isolation, planarization, mechanical protection and support, and the like. High voltage transistors are covered with thick polyimide layers for arc suppression so that the inherently high voltage operation of the device can be achieved. Thick layers of polyimide are also used for alpha particle protection of sensitive MOS integrated circuits with the polyimide layer acting as an alpha particle absorber. Such thick layers, applied after device fabrication is completed, also serve as a mechanical protection layer and as a support for wires connecting the semiconductor die to the die package. Thinner layers of polyimide are used to isolate metal layers in an integrated circuit using multiple layers of metallization. Thin polyimide layers are also used as a final passivation layer on integrated circuits, and to planarize irregular surfaces in integrated circuit fabrication. Other uses of polyimide include insulation on transformer wire, and insulation and isolation on printed circuit boards.
In each application the polyimide is applied as a liquid and must subsequently be cured. The material applied may be a polyimide, a partially imidized material, or a polyimide precursor such as a polyamic acid. Hereafter such materials will be referred to generically as polyimides. The viscosity of the liquid applied and the method of application generally determine the thickness of the resulting layer. To achieve the final cured state, which is a fully imidized state, the material must be heat treated to allow the material to become fully cross-linked. Suppliers of the polyimide material suggest that the material be cured in a series of steps with each step being at a higher temperature than the last. While the exact cure cycle differs slightly from manufacturer to manufacturer, a typical cycle includes about 30 minutes at 135.degree. C., one hour at 300.degree. C., and then 10 minutes at about 400.degree. C. In some applications, however, 400.degree. C., and often even 300.degree. C., is high enough to degrade or even destroy the device or other substrate to which the polyimide is being applied. For example, some solders used in assembling transistors or devices on a printed circuit board melt as low as about 270.degree. C. The standard, advertised curing cycles thus are too high in temperature to permit the use of polyimides in many applications.
The polyimide must be fully cured to be fully effective in its intended use. Partially cured polyimide, for example, is less resistant to etching in certain etchants than is fully cured polyimide. Also, partially cured polyimide is more susceptible to the absorption of water which can lead to long term reliability problems. Further, the partially cured polyimide is less stable than is the fully cured polyimide, and this can lead to other reliability problems.
Unfortunately, there has not been a good way in which to measure polyimide films to determine that they are fully cured. It is conventional to associate degree of cure with the dielectric dissipation factor of the polyimide material. A fully cured layer has a near zero dissipation factor. Numerically, a fully cured layer has a dissipation factor less than about 0.001. The usual way to measure dissipation factor is to apply the polyimide to a metallized glass slide. After curing the polyimide, a second metal electrode is applied to the top of the polyimide and the dissipation factor of the resulting parallel plate capacitor is measured. If it is determined that the curing is not complete, another sample is made up in the same manner, the cure cycle is adjusted, and the dissipation of the new capacitor is measured. In this way, approximate cure cycles are derived. The cure cycle is only approximate since the test capacitor may not be a good approximation of the actual application. In addition, the procedure is very laborious, so that, in practice, it is not often used.
In view of the widespread potential applications for polyimides, the need to cure those polyimides at relatively low temperatures, the need to effect a complete cure of the polyimide, and the need to realistically monitor the cure cycle, it is therefore an object of this invention to provide an improved process for curing a polyimide layer.
It is another object of this invention to provide an improved low temperature polyimide curing process.
It is a still further object of this invention to provide an improved process for monitoring the curing of a polyimide layer on a realistic test structure.
It is yet another object of this invention to provide an improved process for fabricating semiconductor devices.